My mum got around 7 texts messages from a mobile number, which said it was from T-Mobile.

These texts were alerting her that the message was delivered to a landline successfully, and gave the landline number ( which was also the first contact in her phonebook )

Upon checking the sent messages, there were indeed blank messages to the landline in question - now, my mum didnt know you could text landlines, so she wouldnt have done it, and secondly, her phone was in her bag at all times. After the 4th message came through, she gave the phone over to me, and as I was checking her inbox, another message sent while I was doing this ( checked via sent messages).

Now, she phoned T-Mobile this morning, and they are going to open an investigation into why this has happened, as the number which was texting her to tell her the messages were sent, was not a registered T-Mobile number, despite it saying so.

My question is just simply how this could happen, as T-Mobile could not really explain it

Pretty obviously, the phone was unlocked in the bag and while walking along the phone has sent blank messages to the first entry in the phone book. The texts you recieved are telling you that the messages were sent sucessfully. The reason one was sent while you were holding the phone is likely to be messages stored in the outbox ready to send. I bet the person first in the list is pretty worried that thier phone has been ringing off the hook!!!

No scam there as far as I can see? Just the fact of making sure the phone keypad is locked before putting the phone down?

Unless I'm not Reading that right you have nothing to worry about - it's very common.

the thing is, the phone was locked, as it was secure in my mums handbag in the little mobile phone pouch :\ ... and the only way to unlock the phone would be to slide it up, and that would not have been possible within that small space

And, T-Mobile also said the number that the texts were originating from ( the number sending alerts back to us ) was not a registered T-Mobile number

Nothing is impossible in a handbag. I did it once in my pocket -11 calls to the first person in my address list. It's not unheard of.

Anything is possible. The fact of the matter is that if the messages were in the sent box they will definitely have been sent from the phone.

Two contributing factors:
blank messages
first person in address list.

Don't worry about it, doubly make sure the phone is locked. Live and learn.

could be that customer services don't know, could have been BT or VM on behalf of T Mob.

I agree that its far, far more likely that the phone has been casually thrown in the bag.

Otherwise, someone would have to have not only hacked into the phone to send a message, but also into the phone system to send an alert. All to send some blank messages, I think not!

Just tell your mum that phone manufacturers tell you not to put a phone loose in a bag for a reason.

Incidentally, why not phone the number and ask if they have been getting these messages.